1. Field of the Invention
This invention pertains generally to removal of hydrogen sulfide (H2S) in biogas and subsequent removal of Nitrogen oxides (NOx) in engine exhaust, and more particularly to treatment and removal of H2S and NOx in a biogas power system with carbon media and a microwave treatment system.
2. Description of Related Art
There are over 2100 dairies just in California with a potential to produce about 40 million cubic feet per day of biogas with a potential generation capacity of about 136-140 MW. Additionally, anaerobic digesters such as landfills and sewage and food digesters can produce biogas that can be used to make electricity, hot water or other uses. However, contaminants in the biogas limit the ability to fully develop these resources for electric generation such as fuel cells and turbines. Contaminants such as H2S in the biogas also prevent the use of post combustion technologies such as catalysts on engines.
Advanced technologies that could reliably reduce NOx emissions from small (50 kW to 400 kW) reciprocating engines and meet the California Air Resources Board (CARB) 2007 NOx standards for Combined Heat and Power (CHP) distributed generation systems have been researched. Hydrogen sulfide (H2S) produced in the biogas causes significant equipment operation and maintenance issues and restricts power equipment options almost exclusively to reciprocating engines. Sulfur dioxide (SO2) in the engine exhaust, even in small quantities, will poison catalytic emission control systems otherwise suitable for use on natural gas fired engines.
Two emission control strategies may meet the CARB 2007 NOx standards for reciprocating engines; combustion modifications and post combustion control. Lean burn engines use a high air to fuel ratio to lower combustion temperature and NOx formation. This combustion technology was developed for use on large natural gas engines and only recently has been offered on smaller engines. About ten reciprocating engine models below 400 kW are currently available with lean burn technology. Expected NOx emissions range from 0.7 to about 2 g/bhp-hr (without post combustion catalyst). Only four engine manufacturers are known to warrant NOx emissions on lean burn engines smaller than 400 kW fueled by biogas. Three manufacturers indicate that they would guarantee NOx emissions to 1.0 g/bhp-hr without a catalyst.
Five commercially available technologies for post combustion controls are possibilities for use on reciprocating engines burning biogas. These include Oxidation Catalyst, Non-selective Catalytic Reduction (NSCR), Selective Catalytic Reduction (SCR), Selective Non Catalytic Reduction (SNCR) and NOx adsorbents (SCONOX and NOXTECH). The Oxidation Catalyst is used with rich burn engines to remove VOC's and CO and does not remove NOx. The oxidation catalyst, NSCR and SCR are susceptible to Sulfur poisoning and would require reliable and efficient pretreatment of the biogas to remove H2S. SNCR uses urea or ammonia to remove NOx but has not been used successfully on IC engines. SCONOX is a dry absorbent that removes NOx and must be regenerated, but was developed for turbines and has not been used on IC engines. The NOXTECH® system relies on the injection of urea or ammonia into the exhaust of an IC engine and reacts with NOx after heating the exhaust to between 1400-1500 degrees Fahrenheit. Additional fuel and chemicals are needed and the requirement to heat the exhaust complicates installation of heat recovery equipment on the engine exhaust. A small amount of ammonia slip is also reported. Expected NOx emissions for NOXTECH are at 0.3 to 0.5 g/bhp-hr (0.92 to 1.5 lb/MW-hr) which will not meet the 2007 CARB standard of 0.07 lb/MW-hr or the proposed 2008 standard of 0.7 lb/MW-hr for engines operating on digester biogas.
The cost to install and operate these post combustion technologies on small to medium reciprocating engines is significant. Equipment cost coupled with the uncertainty of performance running on biogas makes it difficult to recommend funding for projects without manufacturer assurance of meeting the proposed CARB standards for NOx. A survey of eight engines currently operating on dairy digesters in California shows that all are rich burn engines and that only two are equipped with SCR technology. Both the units with SCR report problems with the catalyst and H2S removal system.
Dairy digesters typically produce about 50%-60% methane, 40%-50% CO2 and sulfur impurities mostly in the form of H2S in the range of 0.06% to 0.2%. Hydrogen Sulfide has a strong order that can be detected at threshold levels of about 0.47 ppb and has an OSHA IDLH level of 300 ppm. Assuming emissions of SOx are not an issue, boilers can tolerate H2S, levels up to 1000 ppm, reciprocating engines about 10 to 100 ppm and fuel cells 10 ppm to 20 ppm.
Reciprocating engines operating on digester biogas compared to natural gas engines cost about 20% more to install and about 80% more to maintain. Sulfur plugs filters, causes deposits on valves and cylinders and contaminates lubricating oil. It has been reported that some operators must change spark plugs frequently ($1000 annually) and change oil as often as weekly ($350 to $1000 per month).
The H2S pretreatment system of choice for most dairy digesters is gas contact with an iron oxide media. The most well known treatment system is an iron sponge. This is a container of iron oxide impregnated media (typically woodchips) that scrubs the inlet gas from the digester. The iron sponge is sized for a residence time of about 60 seconds and the media can collect up to about 2.5 times its weight in sulfur compounds. The media can be partially regenerated by exposure to air or by wetting for about 10 days. Eventually the media must be discarded and replaced with new media. With increasing frequency, the spent media is classified a hazardous waste by local regulators. One example of an iron sponge system costs about $50,000 to install with annual operating costs ranging from $250 to $4000.
Proprietary iron-oxide media such as SulfaTreat®, Sulfur-Rite®, and Media-G2® have been installed as improved alternatives to the iron sponge at a few digester sites. These use different media and additional chemical treatment to remove sulfur. Some of these media have limited regeneration capacity or can be deposited in a landfill. One dairy digester site using Media-G2 has two vessels with about 760 kg of media each with a residence time of about 62 seconds per vessel. Annual media consumption ranges from 1460 kg to about 5900 kg with media replacement costs on the order of $2050 to $8290.
Granulated Activated Carbon (GAC) and other carbon media products are used extensively for filtration of contaminants in water and gas streams. GAC contains micropores that capture and hold many organic and polar molecules and is more effective for larger molecules. In other cases, the carbon acts as a catalyst to drive a reaction with the carbon and the selected molecule in a process known as chemisorption.
Commercially available GAC and pelletized activation carbon (PAC) have the surface area in the range of 800-1000 m2/g. These activated carbon media easily adsorb SO2, NOx, and VOCs. The carbon adsorption capacity is dependent on the composition of gas. The SO2 adsorption capacity is about 5-20 grams per 10 g GAC in the dry gas environment.
The GAC adsorbs most VOCs and is used in removing common solvent vapors used in drying cleaning and parts washing operations. The carbon adsorption capacity is strongly dependent on the VOC molecular weight. The adsorption capacities of toluene and methylene chloride at the room temperature are 20 and 5 g/100 g GAC, respectively. However the adsorption capacity of CH4 in GAC is negligible.
Most GAC adsorber systems use two-stage fixed beds. When the first GAC bed is saturated, the GAC is replaced by fresh GAC. Then, the second bed operated as the first bed until GAC is saturated. The saturated GACs are transported to the regeneration plant or disposed in landfill site.
In engine exhaust gas, NOx is present mostly as NO. When the exhaust gas is cooled, NO is converted into NO2. At 60-170° F. the conversion results in equal quantities of NO and NO2. An important aspect of NOx adsorption is that it must be in NO2 form to adsorb on carbon within any reasonable operating temperatures. Therefore, the conversion of NO to NO2 is critical to NOx removal. However, once NO2 is adsorbed in the GAC, it is no longer available in the equilibrium reaction with NO. With sufficient residence time in the GAC, substantially all NO is converted to NO2 and adsorbed in the GAC. Operational experience and laboratory tests show that GAC will adsorb about 10% by weight NO2 in engine exhaust.
The GAC adsorption capacity for H2S is 5-15% by weight. Therefore, the GAC can be used economically to remove the H2S from the biogas that contains lower concentrations of H2S. Typically GAC is disposed of in a landfill when saturated with H2S.
Impregnating GAC with alkaline or oxide solids enhance the physical adsorptive characteristics of the carbon with chemical reaction. Sodium hydroxide (NaOH), sodium carbonate (Na2CO3), and potassium hydroxide (KOH) are common impregnators. The metal oxide impregnation increases the GAC adsorption capacity significantly. Typically, 20-25% loading by weight of H2S can be achieved, which is 10-15% greater than regular GAC adsorption capacity. The metal-impregnated GAC is almost twice more expensive than GAC. However, the use of metal-impregnated GAC will be more economical for the adsorbers without the on-site carbon reactivation because of its greater adsorption capacity.
Once GAC can no longer adsorb a chemical compound, breakthrough will occur where the contaminant will flow all the way through the bed without being adsorbed. At this point, the GAC is no longer effective and must be replaced. In many cases, such as GAC filled water filters or respirators, the GAC is thrown away and a fresh GAC filter or cartridge is installed. In large scale processes, or where the contaminant can be recovered or destroyed, regeneration of the GAC may be preferred.
There are four processes commonly used for GAC regeneration: Temperature Swing Adsorption (TSA), Pressure Swing Adsorption (PSA) Inert Purge and Displacement Purge. TSA takes place by heating the GAC. With PSA the adsorption takes place at an elevated pressure and regeneration at a lower pressure. Inert gas purge reduces the partial pressure of the adsorbate in the gas phase so that desorption occurs. A purge gas that is more strongly adsorbed than the impurity is used to desorb the original contaminant. Steam regeneration is a combination of TSA and purge. In each process, the contaminant is still present in the purge stream and must be captured, burned or is vented to the atmosphere.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The rotational and vibrational frequencies of the electrons represent the most important frequency range. The electromagnetic frequency spectrum is usually divided into ultrasonic, microwave, and optical regions. The microwave region is from 300 megahertz (MHz) to 300 gigahertz (GHz) and encompasses frequencies used for much communication equipment. For instance, refer to Cook, Microwave Principles and Systems, Prentice-Hall, 1986.
Often the term microwaves or microwave energy is applied to a broad range of radiofrequency energies particularly with respect to the common heating frequencies, 915 MHz and 2450 MHz. The former is often employed in industrial heating applications while the latter is the frequency of the common household microwave oven and therefore represents a good frequency to excite water molecules. In this writing the term “microwave” or “microwaves” is generally employed to represent “radiofrequency energies selected from the range of about 500 to 5000 MHz”, since in a practical sense this large range is employable for the subject invention.
The absorption of microwaves by the energy bands, particularly the vibrational energy levels, of atoms or molecules results in the thermal activation of the nonplasma material and the excitation of valence electrons. The nonplasma nature of these interactions is important for a separate and distinct form of heating employs plasma formed by arc conditions at a high temperature, often more than 3000.degree. F., and at much reduced pressures or vacuum conditions. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Supplementary Volume, pages 599-608, Plasma Technology. In microwave technology, as applied in the subject invention, neither of these conditions is present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable chemical reactions since they can act locally on a microscopic scale by exciting electrons of a group of specific atoms in contrast to normal global heating which raises the bulk temperature. Further this microscopic interaction is favored by polar molecules whose electrons become easily locally excited leading to high chemical activity; however, nonpolar molecules adjacent to such polar molecules are also affected but at a reduced extent. An example is the heating of polar water molecules in a common household microwave oven where the container is of nonpolar material, that is, microwave-passing, and stays relatively cool.
In this sense microwaves are often referred to as a form of catalysis when applied to chemical reaction rates; thus, in this writing the term “microwave catalysis” refers to “the absorption of microwave energy by carbonaceous materials when a simultaneous chemical reaction is occurring” For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.